The outer shell of most compressors is composed of either a low carbon hot or cold rolled steel stamping or gray cast iron. The steel or cast iron, without a corrosion protectant coating, would typically corrode at a fast rate even in a non-marine environment. For conventional compressor applications, the outer surface of the compressor body is painted to minimize corrosion. Corrosion mitigation is important not only to extend the useable life of the compressor, but also to prevent premature failure of the pressurized shell which may result in personal injury.
The steel compressor's outer surface is composed of several stamped steel components that are assembled together primarily by welding. Welding, in itself, causes the surface of the steel be even more prone to corrosion due to several metallurgical factors, two of which are hindering paint adhesion and forming pinholes. The cast iron compressor version is composed of several iron castings assembled together by fasteners. In the case of gray cast iron, corrosion is also prone mainly because of the intrinsic presence of graphite within the cast iron. Graphite encourages corrosion because of the galvanic difference between iron and graphite, which causes preferential corrosion of the iron matrix. Therefore, it is obvious to any expert in the corrosion field that the aforementioned compressor types are highly likely to corrode, especially in extreme environments.
The painting process mentioned as the prior art, has the following sequence of events associated with it's application: Liquid chemical cleaning of the steel or iron surface to remove organic and inorganic contamination, phosphatizing the cleaned surface (creating an iron phosphate layer that aids in the adhesion of the paint), sealing the phosphated coating (sealing controls the phosphating reaction and prepares the surface for painting), painting the compressor (either with a powder electrostatic spraying, dipping or liquid spraying methods), curing the paint either at room temperature or at elevated temperatures.
Typically, the painted compressor must pass several standard test methods to be considered acceptable. ASTMB-117 is one such standard test method. With the paint quality associated with the prior art, it is conceivable that the compressor would pass the standard test methods and still have signs of corrosion of the underlying steel or iron (red rust) visible at localized regions on the painted surface. For most applications, this sporadic red rust is normal and would not affect the functionality of the compressor for the life of the compressor.
However, certain compressor applications require very high reliability and can not succumb to a corrosion failure without great loss. These stringent applications require no visible red rust corrosion on the surface for an extended period of time (as mentioned: despite the fact that it passed ASTM testing). An example of such an application would be climate controlled marine containers that are transported across the ocean. Marine environments are especially corrosion causing because of the presence of salts and other corrosion enhancing constituents found in seawater. The “containers” may be exposed to marine mist or even periodically come in contact with seawater due to splashing. Temperature fluctuations and direct sun light may also be present (which includes the deleterious effect of ultraviolet rays). These containers need to be refrigerated uninterrupted for the entire journey to protect the enclosed cargo. These are high reliability requiring applications, where failure of the compressor would not be easily repairable and would result in large monetary damages if the climate control system ceased to function. This represents an extraordinary challenge considering the especially corrosion inducing marine environment.
The painting procedure described as the prior art does not have a high enough corrosion preventative property associated with it. The prior art, although acceptable for most applications, does not fulfill the requirements of preventing “no visible red rust” during the life of the compressor. The prior art has a weakness in that when nicks or dings occur due to, for example, accidental impact or scratching damage during compressor handling or preventative maintenance, the paint cracks and exposes bare steel which then corrodes at an accelerated rate. The prior art paint process serves only to provide a weak barrier coating. Once this coating is penetrated to the underlying steel, corrosion immediately occurs. Bare metal exposed in this manner will corrode quickly because there is no strong “cathodic protection” provided by the prior art's paint. This is a weakness of the prior art especially because of the long hours the compressors are exposed to corrosive environments.